We don't need to dereference from BDRVNVMeState each time.
Use a NVMeQueuePair pointer on the admin queue.
The nvme_init() becomes easier to review, matching the style
of nvme_add_io_queue().

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201029093306.1063879-16-philmd@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>